Use the terms volume and density to compare gases, liquids and solids in terms of the kinetic molecular theory?
Density is a ratio of mass to volume. So directly, whether a compound is solid, liquid or gas may be related to its density. The most dense phase is the solid phase. The least dense is the gas phase, and the liquid phase is in between the two.
The phase of a compound may be related to the kinetic activity of its constituent atoms or molecules. Energetic molecules by definition show more movement (kinetic), which extends the distance between the molecules. By definition that simultaneously reduces the density.
Sufficient kinetic energy in a compound will result in the molecules moving farther apart, and maintaining a higher movement rate that will exhibit itself as a transition from the solid state to a liquid state.
Further increasing the kinetic energy of molecules in a liquid state will move them farther apart, until they separate into individual molecules in the gas phase.